Fox Fuel Pump Issues After Replacement/Upgrade

Ryu · 2026-06-23T16:42:09-0500

I've got a 93 LX 5.0 that I just replaced the fuel pump (after some quick research stock OEM fuel pump should be 88 LPH) to a 155 LPH from LMR, before install everything was "fine" (and I only say fine because I'm trying to fix a different issue by replacing the fuel pump in the first place) after install while driving it hard I would lose about 60-80% of power with the accelerator pressed to the floor usually after 2k rpm for 1st gear 2.5-3k rpm for 2nd and so on. I reinstalled the old fuel pump and the issue went away so i don't believe I installed the 155 fuel pump incorrectly or pinched any fuel lines nor do i believe there was air in the fuel lines because after reinstalling the old fuel pump I had no fuel delivery issues in the first place. doing more research I saw that for a 255 LPH pump the person who did that to their 92 coup also needed to install a larger guage wire to handle the volts needed to run that pump, do i need to do something similar? what should I do from here? becasue LMR doesn't offer a stock fuel pump

This is the kit I installed https://lmr.com/item/LRS-9350A/1986-97-Mustang-155-Lph-Walbro-Fuel-Pump

TL;DR:

my stock 1993 Mustang LX 5.0 ran normally before the fuel pump swap.
I replaced the factory ~88 LPH pump with a 155 LPH pump from LMR.
After the swap, the car would fall on its face under heavy throttle, losing roughly 60–80% of its power above ~2,000 RPM in 1st gear and ~2,500–3,000 RPM in higher gears.
Reinstalling the original fuel pump immediately fixed the issue.
Because the problem disappeared when the old pump went back in, I don't think the installation was incorrect, a fuel line was pinched, or air was trapped in the system.
I've read that some people running 255 LPH pumps on Fox-body Mustangs upgraded the fuel pump wiring to handle the increased current draw.
My question is whether a 155 LPH pump also requires a wiring upgrade, or if something else could be causing the power loss. or just something im missing completly

General karthief · 2026-06-23T21:46:29-0500

Ryu said:
I'm trying to fix a different issue by replacing the fuel pump in the first place)

What's the issue?

Shakerhood · 2026-06-23T22:54:17-0500

Do you have a 3G Alternator upgrade already?

Ryu · 2026-06-23T23:06:18-0500

General karthief said:
What's the issue?

Engine Post in thread 'Throwing codes 94, 33, 34, 44, and sporadic engine shutoff.'

Oct 13, 2024

89ripper said:
That was a long read! Can you explain in detail what happens with your sporadic shutoffs? Is it right away, or after driving awhile? Does it start right up or hard to start? We will go over you error codes after you follow up.

It’ll only die after driving for 45 min to an hour, only if it’s hot over 75 degrees. When it dies theres a loss of power that happens over 7 seconds as the rpm’s go lower and lower if you slam on the accelerator rpm’s will negligibly rise giving no additional power to the wheels and the engine will inevitably die while driving on the highway going 75...

Ryu · 2026-06-23T23:12:42-0500

Shakerhood said:
Do you have a 3G Alternator upgrade already?

Yea I got this https://lmr.com/item/SVE-17046K2-B/mustang-130-amp-alternator-upgrade-sve-86-93-black one installed

gkomo · 2026-06-23T23:29:22-0500

Yes for a 255lph you should run a wiring upgrade kit. I have a 255lph walboro pump and am running my stock wiring. But, I am running the risk of not getting the full potential of the pump and worst case scenario could maybe melt some wires and start a fire? Not sure but that’s why I have an aeromotive wiring kit sitting in a box waiting to be installed. For a 155lph pump it’s ‘probably’ not needed but still a good idea anyways. That’s probably not your problem.

Maybe you got a defective pump? Have you done any other troubleshooting or just swapped pumps back?

Ryu · 2026-06-23T23:41:17-0500

gkomo said:
Yes for a 255lph you should run a wiring upgrade kit. I have a 255lph walboro pump and am running my stock wiring. But, I am running the risk of not getting the full potential of the pump and worst case scenario could maybe melt some wires and start a fire? Not sure but that’s why I have an aeromotive wiring kit sitting in a box waiting to be installed. For a 155lph pump it’s ‘probably’ not needed but still a good idea anyways. That’s probably not your problem.

Maybe you got a defective pump? Have you done any other troubleshooting or just swapped pumps back?

I hate that these things come defective more often then not just in general. But that's my guess too but and I didn't even know how else to troubleshoot besides the reinstall of the original fuel pump not that I even want the 155 lph pump with current gas prices anyway. I'm looking at getting just a stock replacement on rock auto but before I did i wanted to see if maybe I missed some simple fix with the 155.

AeroCoupe · 2026-06-24T06:41:45-0500

I’m going to say you got a bad pump. I ran a 255 lph pump on stock working for years with no issues. When I moved up in hp is when I started to see fuel delivery issues which is when I reworked the pump. It ensured I had a more constant voltage to the pump.

A 155 or 190 lph pump does not need upgraded wiring.

It’s pretty simple, old pump doesn’t exhibit the problems new one does so new pump is the issue. Try another pump but make sure it’s a genuine Walbro or Aeromotive.

Fuel pump has nothing to do with gas mileage. Engine will only burn what it needs hence the return system.

Mustang5L5 · 2026-06-24T10:09:58-0500

155 does not need upgraded wiring. I ran a 190 for years on stock wiring without any issues (that weren't of my own doing).

It might be in your best interest to get a fuel pressure gauge and actually monitor pressure during these events to determine if this is fuel related or other. Granted if swapping back to stock pump fixes it, it's likly a bad pump.

Fox Fuel Pump Issues After Replacement/Upgrade

Ryu

Member

General karthief

wonder how much it would cost to ship you a pair

Shakerhood

Ryu

Member

Engine Post in thread 'Throwing codes 94, 33, 34, 44, and sporadic engine shutoff.'

Ryu

Member

gkomo

now i can hopefully expect to receive the shaft

Ryu

Member

AeroCoupe

lube between the nut and the face. I know my lubes

Mustang5L5

That is…until I whipped out my Bissell

Similar threads

Fox Fuel Pump Issues After Replacement/Upgrade

Member

wonder how much it would cost to ship you a pair

Member

Engine Post in thread 'Throwing codes 94, 33, 34, 44, and sporadic engine shutoff.'

Member

now i can hopefully expect to receive the shaft

Member

lube between the nut and the face. I know my lubes

That is…until I whipped out my Bissell

Similar threads

Privacy & Transparency

Privacy & Transparency